$-\dfrac{7}{8} - \dfrac{2}{4} = {?}$
$ = - {\dfrac{7 \times 1}{8 \times 1}} - {\dfrac{2 \times 2}{4 \times 2}} $ $ = - {\dfrac{7}{8}} - {\dfrac{4}{8}} $ $ = - \dfrac{{7} - {4}}{8} $ $ = -\dfrac{11}{8}$